The present invention relates to free piston internal combustion engines, and, more particularly, to a free piston internal combustion engine having a hydraulic output and one or more accumulators.
Internal combustion engines typically include a plurality of pistons which are disposed within a plurality of corresponding combustion cylinders. Each of the pistons is pivotally connected to one end of a piston rod, which in turn is pivotally connected at the other end thereof with a common crank shaft. The relative axial displacement of each piston between a top dead center (TDC) position and a bottom dead center (BDC) position is determined by the angular orientation of the crank arm on the crankshaft with which each piston is connected.
A free piston internal combustion engine (FPE) likewise includes a plurality of pistons which are reciprocally disposed in a plurality of corresponding combustion cylinders. However, the pistons are not interconnected with each other through the use of a common crankshaft. Rather, each piston is typically rigidly connected with a plunger rod which is used to provide some type of work output. In a free piston engine with a hydraulic output, the plunger is used to pump hydraulic fluid which can be used for a particular application. Typically, the housing which defines a combustion cylinder also defines a hydraulic cylinder in which the plunger is disposed. The combustion cylinder has the largest diameter; and the hydraulic cylinder has the smaller diameter. The high pressure hydraulic accumulator which is fluidly connected with the hydraulic cylinder is pressurized through the reciprocating movement of the plunger during operation of the free piston engine. An additional hydraulic accumulator is selectively interconnected with the area in the hydraulic cylinder to exert a relatively high axial pressure against the compression head and thereby move the piston head toward the TDC position.
A free piston engine as described above is typically coupled with a hydraulic transformer which typically converts a high flow rate, low pressure hydraulic fluid to a low flow rate, high pressure hydraulic output, or vice versa. The hydraulic output from the hydraulic transformer is utilized to drive one or more components such as a pump within a work unit, such as a vehicle. An example of a hydraulic transformer as described above is disclosed in U.S. Pat. No. 5,878,649, (Raab), which is assigned to the assignee of the present invention.
A problem with utilizing an intermediary hydraulic pressure transformer as described above is that they inherently absorb some of the energy from the system and thereby render the system less efficient. Other methods of converting the hydraulic output power from the free piston engine to a rotating mechanical output power are also known and utilized. However, each of these intermediary power conversion techniques absorb energy from the system and render the system less efficient.
The present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the invention, a free piston engine system is provided with at least one hydraulic pump, each pump having a first fluid port and a second fluid port. A free piston internal combustion engine includes a combustion cylinder, a hydraulic cylinder, a piston reciprocally disposed within the combustion cylinder, and a plunger attached to the piston and disposed within the hydraulic cylinder. A low pressure accumulator is fluidly coupled with the hydraulic cylinder. A first control valve interconnects the low pressure accumulator with the hydraulic cylinder. At least one high pressure accumulator is fluidly coupled with the hydraulic cylinder. At least one second control valve is provided, with each second control valve interconnecting a respective high pressure accumulator with the hydraulic cylinder. A third control valve interconnects the hydraulic cylinder with the first fluid port of each pump. A fourth control valve interconnects the hydraulic cylinder with the second fluid port of each pump. A first working pressure accumulator is coupled between each pump and the third control valve or fourth control valve.
In another aspect of the invention, a method of operating a free piston engine system is provided with the steps of: providing at least one hydraulic pump, each pump having a first fluid port and a second fluid port; providing a free piston internal combustion engine including a combustion cylinder, a hydraulic cylinder, a piston reciprocally disposed within the combustion cylinder, and a plunger attached to the piston and disposed within the hydraulic cylinder; fluidly coupling a first control valve between a low pressure accumulator and the hydraulic cylinder; fluidly coupling a second control valve between a high pressure accumulator and the hydraulic cylinder; fluidly coupling a third control valve between the hydraulic cylinder and the first fluid port of each pump; fluidly coupling a fourth control valve between the hydraulic cylinder and the second fluid port of each pump; coupling a first working pressure accumulator between each pump and one of the third control valve and the fourth control valve; and selectively controlling the first control valve, the second control valve, the third control valve and the fourth control valve to drive at least one pump in a closed flow path.